1. Field of the Invention
Example embodiments of the present invention relate generally to communications systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
FIG. 1 illustrates a conventional Code Division Multiple Access (CDMA) 100. The CDMA system includes a plurality of user equipments (UEs) 105 in communication with one or more serving Node Bs 120/125 over an air interface. The plurality of Node Bs are connected to a radio network controller (RNC) 130 with a wired interface. Alternatively, while not shown in FIG. 1, the functionality of both the RNC 130 and Node Bs 120/125 (alternatively referred to as “base stations”) may be collapsed into a single entity referred to as a “base station router”. The RNC 130 accesses an internet 160 through a gateway support node (GSN) 150 and/or accesses a public switched telephone network (PSTN) 170 through a mobile switching center (MSC) 140.
Referring to FIG. 1, in the CDMA system 100, a power control mechanism is typically used to minimize power consumption and interference while maintaining a desired level of performance. Conventionally, this power control mechanism is implemented with two power control loops. The first power control loop (often referred to as an “inner” power control loop, or “inner loop”) adjusts the transmit power to each mobile station or UE 105/110 such that the signal quality of the transmission received at the UE receiver (e.g., as measured by a signal-to-noise ratio) is maintained at a target signal-to-interference+noise (SINR) ratio, or target Eb/N0. The target SINR or Eb/N0, where Eb is the energy per information bit, and N0 is the power spectral density of the interference seen by the receiver, is often referred to as a power control set point, or threshold. The second power control loop (often referred to as an “outer” power control loop, or “outer loop”) adjusts the threshold such that the desired level of performance, e.g., as measured by a particular target block error rate (BLER), frame error rate (FER), or bit error rate (BER) for example, is maintained.
For example, for link (e.g., forward link or reverse link) power control, the inner loop compares a measured SINR or Eb/N0 of the received signal to the target SINR or target threshold. The SINR of the received signal is periodically measured, for example, at 1.25 ms interval. If the measured SINR or Eb/N0 is smaller than the threshold, there may be too many decoding errors when the receiver is decoding frames of a received transmission, such that the FER is outside an acceptable range (i.e., too high). Accordingly, the receiver requests an increase in power on the link. If the measured SINR or Eb/N0 is larger than the threshold, the receiver requests a decrease in power on the link. Here, the decoded transmission may contain little or no errors, thus the system may be too efficient (FER is far below the acceptable range) and transmit power is being wasted.
The outer loop surrounds the inner loop and operates at a much lower rate than the inner loop, such as at 20 ms intervals, for example. The outer loop maintains the quality of service (QoS) of the link. The outer loop establishes and updates the SINR threshold, which is responsive to changing channel/environmental conditions. The outer loop looks at quality of the link, and if the quality is too poor, the outer loop increases the threshold accordingly. Alternatively, if the link quality is too good, (e.g., an FER less than a target FER of about 1% voice transmissions, higher for data transmissions), the outer loop readjusts the threshold so as not to unduly waste system resources. In view of this, the target SINR is said to be adaptive. And, because this process is performed for each link, each receiver has its own adaptive target SINR such that the target SINRs of different receivers (e.g., UE receivers) differ.
FIG. 2 illustrates a conventional inner loop CDMA reverse link power control process. The process of FIG. 2 is described below as performed with respect to the reverse link from the UE 105 to the Node B 120. However, it is understood that the process of FIG. 2 is representative of a conventional CDMA reverse link power control between any UE in connection with any Node B.
Referring to FIG. 2, at the inner loop, the Node B (e.g., Node B 120) measures the SINR for pilot transmissions received from a UE (e.g., UE 105) in step S105. The measured SINR measurement (step S105) is either a pre- or post-interference cancellation (IC) measurement. In an example, if the measurement of the pilot SINR is performed with post-interference cancellation, the Node B 120 measures the pilot SINR prior to interference cancellation, and then measures the residual interference-to-total interference ratio after the interference cancellation. The ratio of these two quantities is a measure of the post-interference cancellation SINR.
The Node B 120 compares the measured pilot SINR with an adaptive target SINR in step S110. The adaptive SINR target is previously set by the outer loop at the RNC 130 so as to satisfy a level of Quality of Service (QoS), reflected by an expected packet error rate (PER) or FER, for each served UE (e.g., UE 105, 120, etc.). The adaptive SINR target is not the only factor affecting the QoS, however, and the adaptive SINR is set with a consideration of such other factors so as to more accurately tune to the desired level of QoS. For example, another factor potentially affecting the QoS is a traffic-to-pilot ratio (TPR) at the UE 105. The TPR at the UE 105 is fixed, and does not “adapt” as described above with respect to the adaptive target SINR. Here, “fixed” TPR means that, for a given transfer rate, the TPR is set to a constant value and does not change.
The Node B 120 sends a transmit power control (TPC) bit to the UE 105 in step S115. A TPC bit is a single bit binary indicator, which is set to a first logic level (e.g., a higher logic level or “1”) to instruct a UE (e.g., UE 105) to increase transmission power by a fixed amount and a second logic level (e.g., a lower logic level or “0”) to instruct a UE (e.g., UE 105) to decrease transmission power by the fixed amount. In an example, if the comparison of step S110 indicates that the measured pilot SINR is less than the adaptive target SINR, the Node B 120 sends a TPC bit having the first logic level (e.g., a higher logic level or “1”) to the UE 105. Otherwise, the Node B 120 sends a TPC bit having the second logic level (e.g., a lower logic level or “0”) to the UE 105. After the Node B 120 sends the TPC bit to the UE 105 in step S115, the process returns to step S105.
In a further example, the frequency at which the Node B 120 measures (step S105) the pilot SINR, compares the measured pilot SINR with the adaptive target SINR (step S110) and sends TPC bits (step S115) may be based on a desired “tightness” of power control as determined by a system engineer.
While the process of FIG. 2 is being performed at the Node B 120, at the outer loop, the RNC 130 periodically determines whether to adjust the adaptive target SINR based on an analysis of the inner loop communications. This determination may be based on a number of criteria. For example, the RNC 130 decreases the adaptive target SINR if the PER or FER is relatively low (e.g., very few non-acknowledgments (NACKs) are sent to the UE 105 indicating failed transmissions) so as to satisfy a given level of QoS. In another example, the RNC 130 increases the adaptive target SINR if the PER is relatively high (e.g., too many NACKs are being sent to the UE 105) so as to satisfy a given level of QOS. The RNC 130 then updates the adaptive target SINR used by the Node B 120 in the process of FIG. 2 in accordance with the determined adjustment.